The 80% limit is a feature introduced in iPhone 15 models as part of the battery health and charging options. When you choose the 80% Limit, your iPhone will charge up to about 80 percent and then stop charging. This is designed to reduce the wear on your battery and improve its lifespan by reducing the time your iPhone spends fully charged.
However, if the battery charge level gets down to 75 percent, charging will resume until your battery charge level reaches about 80 percent again. This is to ensure that your iPhone has enough charge for your needs.
In addition, with the 80% Limit enabled, your iPhone will occasionally charge to 100 percent. This is done to maintain accurate battery state-of-charge estimates.
So the bottom line is, the phone will sometimes charge it passed 80% just to calibrate and maintain the battery health accuracy of the phone status.
This is the answer I got when I ask apple support. Hope this help